The present invention broadly relates to improved lubricated metallic container stocks and to a novel method of preparing the same. In still another of its aspects, the invention further relates to a novel method of applying an organic coating on lubricated metallic container stocks.
A thin coating of a lubricant is applied on the surfaces of most metallic container stocks produced in this country. The lubricant is necessary in order to facilitate handling of sheets of the product and to protect the surfaces against abrasion, scratches, and to some extent corrosion. If the lubricant is not applied, the individual sheets tend to stick together and do not handle well in subsequent fabricating steps utilizing automatic machines for applying organic coatings, lithographing and manufacturing containers.
The amount of lubricant that is applied is quite small and must be maintained within a relatively narrow critical range. If too little lubricant is applied, then the disadvantages enumerated above when no lubricant is applied are present to some extent. If too much lubricant is applied, there is excessive slippage and it is difficult to stack and handle the individual sheets. Excess oil may also adversely affect the application of organic coatings, lithographing, labeling and other subsequent steps. The lubricant usually should be applied in an amount of about 0.05-1.0 gram per base box. The term "base box" is used in this art to refer to a quantity of metallic container stock having a total surface area, i.e., the surface area of both sides, of 62,720 square inches. The term "base box" is used hereinafter for convenience in disclosing and claiming the invention and refers to a total surface area of 62,720 square inches on which the lubricant is applied.
Lubricants may be applied to metallic container stock by a number of methods such as branning, dipping in a solvent solution of the lubricant followed by evaporating the solvent, and electrostatic deposition. Electrostatic deposition is a method that is suitable for the application of the small quantities of lubricant set out above in the form of a thin uniform film at high line speeds. The line speeds employed in the manufacture of metallic container stocks sometimes vary from 250 feet per minute to as high as 2,000 feet per minute and a method capable of control of the rate of lubricant application for these line speed variations is required. As a result, the lubricant that is selected for commercial use should be capable of being applied by electrostatic deposition.
A lubricant for lubricating metallic container stocks should possess a unique combination of properties. If any one of this combination of properties is missing, then the lubricant is not entirely satisfactory. As a result, only a very small percentage of the numerous potential lubricants are satisfactory. The problem of selecting a suitable lubricant is complicated by the fact that it is often impossible to predict whether or not a specific substance will be suitable by laboratory scale lubricating experiments and the potential lubricant must be applied in a production run.
Some of the more important characteristics of an entirely satisfactory lubricant for metallic container stocks are as follows:
(1) In the case of electrostatic deposition, the lubricant should be easily atomized to form a finely divided dispersion thereof in a gaseous medium such as air. A carefully controlled quantity of the suspended particles of lubricant in the resultant gaseous dispersion should be capable of being electrostatically precipitated in the form of a thin uniform film upon metallic container stock moving at high line speeds.
(2) The lubricant should be retained on the container stock for a reasonable period of time in an amount effective to lubricate the surface.
(3) The lubricant should be compatible with protective and/or decorative organic coatings which are subsequently applied to the lubricated container stock. The lubricant also should have no effect or a beneficial effect on the wetting of the surface area to be coated and the adhesion thereto of the protective and/or decorative organic coatings.
(4) The lubricant should not be discolored upon heating or baking the lubricated container stock at a temperature sufficiently high to harden or cure the organic coatings.
(5) The film of lubricant should be resistant to oxidation and not harden or lose its desirable lubricant properties over a reasonable period of time.
(6) The lubricant should be free of objectionable tastes and odors in the quantities applied and under the conditions of use.
(7) The lubricant must be pharmacologically safe and nontoxic in the amounts applied as the lubricated container stocks are often used to manufacture containers for preserving and storing foodstuffs.
A number of naturally occurring vegetable oils and synthetic esters of carboxylic acids have been proposed heretofore for use in lubricating metallic container stocks. Vegetable oils such as cotton seed oil, palm oil and the like have a tendency to oxidize to a solid film which is no longer a good lubricant after a relatively short period of storage. A number of the synthetic ester lubricants have this deficiency, and as a result of this and other advantages, several of the esters of sebacic acid have been used heretofore as lubricants. Synthetic esters of sebacic acid prepared from alcohols containing eight or more carbon atoms, such as dioctyl sebacate, are widely used as lubricants at the present time. While dioctyl sebacate is one of the best synthetic ester lubricants available heretofore, it does have some deficiencies in view of recent developments in the art. For example, a substantial percentage of the metallic container stock produced in this country is given a cathodic dichromate treatment such as disclosed in U.S. Pats. No. 3,278,401 and 3,296,106. The cathodic dichromate treatment increases corrosion resistance and is desirable for this purpose. When the cathodic dichromate treated container stock is lubricated with dioctyl sebacate or other prior art lubricants, wetting or eyeholing problems often occur. This tendency is especially pronounced when epoxy type lacquers and certain other advanced organic coatings are used. As a result of this deficiency, the art has long sought a suitable lubricant which meets all of the requirements set out above, regardless of the type of organic coating and whether or not the container stock has received a cathodic dichromate treatment. However, an entirely satisfactory lubricant was not available prior to the present invention.
It is an object of the present invention to provide an improved lubricated metallic container stock which is readily wetted by initially fluid organic coating materials which often present wetting problems such as epoxy lacquers.
It is a further object to provide a novel method of lubricating metallic container stocks wherein the lubricant is an ester of citric acid.
It is still a further object to provide a novel method of applying an organic coating on lubricated container stocks wherein the lubricated surface area to be coated is easily wetted by initially fluid organic coating materials and whereby eyeholing and other imperfections in the final hardened organic coating may be minimized.
Still other objects and advantages of the present invention will be apparent to those skilled in the art upon reference to the following detailed description.